1. Technical Field
This application relates to computing devices, and more particularly to the field of managing storage for computing devices.
2. Description of Related Art
Host processor systems may store and retrieve data using storage devices containing a plurality of host interface units (host adapters), disk drives, and disk interface units (disk adapters). Such storage devices are provided, for example, by EMC Corporation of Hopkinton, Mass. and disclosed in U.S. Pat. No. 5,206,939 to Yanai et al., U.S. Pat. No. 5,778,394 to Galtzur et al., U.S. Pat. No. 5,845,147 to Vishlitzky et al., and U.S. Pat. No. 5,857,208 to Ofek. The host systems access the storage device through a plurality of channels provided therewith. Host systems provide data and access control information through the channels of the storage device and the storage device provides data to the host systems also through the channels. The host systems do not address the disk drives of the storage device directly, but rather, access what appears to the host systems as a plurality of logical volumes. The logical volumes may or may not correspond to the actual disk drives.
It is desirable to be able to quickly get a consistent snapshot of data of a logical volume. Snapshot functionality may be provided on a storage device using protection bits to indicate when a track needs to be copied to a target for the snapshot. A session corresponding to the protection bit may be established so that when a protection bit is set, a write operation to a track of the storage device is intercepted before the write operation modifies the track. The track is first copied to the target(s) of the snapshot before the write operation continues. Such a copy is called “copy-on-first-write” (COFW). After the copy to the target has taken place, the protection bit is cleared so that subsequent writes do not need to be intercepted in the same manner.
The impact of COFW to the response time may be substantial since the write operation to the storage device is paused until the copy to the target is complete. Accordingly, it is desirable to improve the response time of the first write operation to a track of a storage device after a snapshot (or similar) operation is initiated for the storage device.